We have recently developed an electrophoretic assay for human tryptophanyl-tRNA synthetase in rodent-human hybrid cells. The technique is readily adaptable to other aminoacyl-tRNA synthetases and to certan other enzymes involved in tRNA metabolism and protein synthesis. We propose to screen rodent and human aminoacyl-tRNA synthetases for electrophoretic differences, and to use the differences observed to map genes for the enzymes using rodent-human hybrid somatic cells. To supplement electrophoretic assays, we propose to elicit antisera by injecting mice and rabbits with highly-purified aminoacyl-tRNA synthetases from human placenta. Enzymes will be tested for species-specificity and adsorbed with mouse cell or tissue extracts to remove cross-reacting antibodies. Species-specific antisera will be used to assay for the presence of particular human aminoacyl-tRNA synthetases in rodent-human somatic cell hybrids. Despite increasing evidence that certain mammalian aminoacyl-tRNA synthetases are aggregated, direct information concerning the structure of the aggregates is lacking. Enzymes which methylate tRNA, and certain translational elongation and initiation factors also occur in aggregates. The structural relationship between the various aggregated enzymes is unknown. Antisera elicited to purified human aminoacyl-tRNA synthetases will be used to determine whether aminoacyl-tRNA synthetases with distinct amino acid specificities are physically associated with each other, with tRNA methyltransferases, with tRNA-CCA terminal transferases, or elongation factor II. It has recently been demonstrated that many SDS-treated proteins elicit antibodies in rabbits which specifically recognize the native antigen. Antibodies will be raised to SDS-treated aminocyl-tRNA synthetases, and tested for binding, inhibition, and precipitation of the native enzymes, and for species-specificity.